Duchenne muscular dystrophy (DMD) is an X-linked recessive, lethal disorder caused by mutations in the dystrophin gene. Considerable progress has been made both in understanding the function of dystrophin, and in demonstrating the feasibility of gene therapy for DMD. Nonetheless, numerous obstacles remain before gene therapy can be effectively applied to this common genetic disease. These obstacles include a lack of data on the reversibility of the dystrophic pathology, limited ability of viral vectors to carry the enormous dystrophin gene or cDNA, and questions about the effectiveness of inefficient delivery methods of dystrophin vectors. This application proposes to address these concerns by generating several novel strains of transgenic mice. The ability to modulate the dystrophic phenotype will also be explored using viral delivery of dystrophin and several death protectors to mdx mice, a model for DMD. Transgenic mice that express moderate levels of dystrophin are able to prevent the development of dystrophy in the mdx mouse, a model for DMD. Delivery of adenoviral vectors expressing truncated dystrophins to neonatal, immune tolerant mice can also prevent muscular dystrophy near the site of injection. However, it has not been possible to demonstrate that the pathology can be halted or reversed in adult, dystrophic animals. Aim1 will address the feasibility of reversing muscular dystrophy at different stages of the disease by studying a transgenic mouse line that displays tetracycline-inducible dystrophin expression. Aim 2 will continue previous work aimed at understanding the structural basis of dystrophin functional domains, with the goal of developing severely truncated cDNAs that can be carried by a variety of promising viral vectors, such as adenoassociated viruses (AAV). Currently, the only vectors capable of carrying the full-length dystropin cDNA have problems with cytotoxicity, immune rejection or low titers. AAV efficiently infect muscle with no immune response, but have a limited cloning capacity. Aim 3 explores the ability to modulate dysrtophy by delivery of dystrophin with proteins that repress apoptosis and/or enhance muscle regeneration. Achieving uniform and efficient gene delivery to muscles using viral vectors is a daunting goal. The ability to modulate dystrophy and prolong muscle fiber longevity could greatly facilitate the effectiveness of dystrophin gene replacement strategies. These studies will provide new insights into both the structure of dystrophin and the mechanisms of dystrophic cell death and will help advance the development of gene therapy.